Although a number of different metals may be used in the practice of the present process, as hereinafter disclosed, the description of the background and general application of the invention is made with respect to the preferred metal, tin.
Hydrocarbyl substituted tin halides, such as methyl tin chloride, have been found particularly useful in the preparation of derivatives serving as stabilizers for halogenated organic materials, such as polyvinyl chloride resins, chlorinated paraffins, and the like. High purity trialkyl tin compounds including the chlorides are also effective as biocidal compositions.
Various methods have been suggested to prepare hydrocarbyl tin chlorides. To data none has been completely satisfactory. For example, methyl tin chlorides have been prepared commercially by the reaction of Grignard reagent with tin tetrachloride, a process which is expensive and inconvenient to carry out. The Grignard reaction, for instance, produces undesirable side effects. In another process, methyl chloride is bubbled as a vapor through a pool of molten tin or tin alloy held in a vertical reaction tube while heating the tube to a temperature sufficiently high to effect the reaction. This process obviously requires relatively high temperatures, at least higher than that needed to maintain the tin or its alloy in a molten state. At reaction temperatures above about 400.degree.C, alkyl halides begin to decompose thermally. U.S. Pat. No. 2,679,506 also teaches a process for preparing dimethyl tin dichloride in which molten tin is used and at comparatively high temperatures.
Many compounds have been tested as catalysts in a reaction for preparing hydrocarbyl tin halides but have not been found to give entirely satisfactory results. For instance, such compounds are not sufficiently general in their reaction, or provide an insufficient yield of the desired product, or contaminate the hydrocarbyl tin halide with undesirable catalyst residues. Still other processes use expensive catalysts, such as the quarternary onium iodides, and form substantial amounts of by-products. Further, in some prior processes, the conversion rate of tin is rather low, and it is difficult to obtain the desired hydrocarbyl tin halide in a sufficient pure state.